1. Field of Invention
This invention relates to analog-to-digital converters, and more particularly to sigma-delta type analog-to-digital converters implemented with superconducting circuit elements arranged in a two loop configuration.
2. Background Information
Sigma-delta converters are a class of analog-to-digital converters. The basic sigma-delta converter utilizes an integrator to which the analog signal to be converted is applied. A single rough comparator operating at high speed converts the output of the integrator to a single bit digital signal. The comparator is combined with a digital-to-analog converter to form a quantizer. A digital filter converts the high speed single bit output of the quantizer into a multi-bit digital output. The converter also includes a feedback loop including the comparator, the digital-to-analog converter and the integrator. The quantizer samples the integrator output at a rate many times the Nyquist rate. The rate of the multi-bit output signal is a function of the sampling rate and the number of bits in the output signal. The feedback in the sigma-delta converter integrates the error in the least significant bit thereby shifting quantization noise, which limits the dynamic range in conventional analog-to-digital converters, to frequencies above the frequency of the analog input signal. The chief advantage of sigma-delta converters is that they substitute high speed digital signal processing for the high precision analog circuits required in conventional analog-to-digital converters.
The basic sigma-delta converter has a single loop. Higher order converters have additional feedback loops, each with an integrator. .A two loop sigma-delta converter has an outer loop which integrates the difference between the analog input signal and the quantizer output and applies the integrated error as the input to the inner loop. Such a two loop converter substantially reduces the sampling rate required for a given resolution of the digital output signal.
Typically, single loop and higher order sigma-delta converters have been implemented in semi-conductor integrated circuits. Such semi-conductor versions are used for example in high performance systems to obtain 18 to 20 bits on the 22 KHz audio band. This is accomplished with megahertz sampling of the analog signal.
Our commonly owned Patent Application Ser. No. 07/710,856 filed on Jun. 6, 1991, now U.S. Pat. No. 5,140,324, discloses a sigma-delta converter implemented with superconducting elements and operating with GHz sampling rates to provide high resolution for megahertz signals. This superconducting sigma-delta converter utilizes a superconducting inductor as the integrator, a Josephson junction as the quantizer and a superconducting Quantum Interference Device (SQUID) to generate GHz rate sampling pulses. When the sum of a sampling pulse and the inductor current generated by the superconducting inductor exceeds a critical current, the Josephson junction generates a voltage pulse which represents a digital "one" output. The voltage pulse also provides feedback to the superconducting inductor. This feedback is very precise and stable as each voltage pulse generated by the Josephson junction is a flux quantum.
Sigma-delta converters having sampling rates in the GHz range are useful in generating high resolution digital signals from high frequency analog signals in applications such as for example surveillance radar, signal interception and medical imaging.
It would be desirable to have a sigma-delta converter operating in the GHz and above sampling range which would require less oversampling and therefore could provide better resolution and/or convert higher frequency analog signals.